Fractured-vuggy Reservoirs Research Articles

Production optimization for water flooding in fractured-vuggy carbonate reservoir – From laboratory physical model to reservoir operation

Conventional physical model experiments of carbonate reservoirs are limited to the investigation of production patterns and mechanisms for a single well or a well group, and cannot effectively optimize the oil production at the reservoir scale. For an actual fractured-vuggy reservoir in Tahe Oilfield, China, seismic and drilling data were first analyzed to obtain the distributions of fractures and vugs, and the dynamic tracer monitoring was used to determine the type of connection between fracture-vug units. Then, according to shape similarity, the real fracture-vug units are simplified into 12 visualized physical models made of transparent acrylic plates with a scale of 300:1. Different physical models were further combined together with the connectivity modes derived from dynamic tracer monitoring, thus embodying the entire complex carbonate reservoir to a laboratory-scale physical model for the first time. Experiments were then carried out with these models and results showed that changing the injection and production parameters and increasing the number of flow channels between injector and producer have little effect on the displacement efficiency, while altering the water flow direction and converting injector into producer achieved better effect, due to the increasing fracture-vug units that can be displaced. In the field production, the output of TK652, TK605CH and TK608 wells increased from 11, 5, 1 m3/d to 18, 18, 19 m3/d, respectively, by closing well TK659 and converting well TK6100 (two high water-cut producers) to change the direction of water flow. The laboratory physical modeling technique illustrated in this study provides a simplified and unified way for the production optimization of fractured-vuggy carbonate reservoirs.

Two-Phase Modeling within Fractured Vuggy Reservoir

The paper is devoted to the mathematical modeling of flows within discrete fracture network systems, taking into account the fluid flow along vugs associated with fracture intersections. A mathematical model is described for a two-phase flow accounting for the flow exchange between fractures and vugs, as well as capillary and gravitational forces. Capillary forces are characterized by the Young–Laplace model which takes into account rock wettability, surface tension, fracture aperture, and vug cross-section diameters. The appropriate numerical simulation techniques are presented. Under various wettability conditions, the effect the flow within vugs has on the water–oil displacement process in a fractured vuggy reservoir is numerically investigated. It is shown that the presence of vugs at the intersections of fractures may significantly influence the flow characteristics under certain conditions.

Analysis of Gas Flow Behavior for Highly Deviated Wells in Naturally Fractured-Vuggy Carbonate Gas Reservoirs

To improve the carbonate gas reservoir development and production, highly deviated wells (HDW) are widely used in the field. Production decline analysis of HDW is crucial for long-term gas reservoir development. However, it is a new challenge to incorporate the complex pore structure of naturally fractured-vuggy carbonate gas reservoirs and evaluate the production performance of HDW. This paper presents a semianalytical model to analyze the pressure and production behavior of HDW in naturally fractured-vuggy carbonate gas reservoirs, which consist of fractures, vugs, and matrix. The primary flow occurs only through the fracture and the outer boundary is closed. Introducing pseudopressure and pseudotime, the Laplace transformation, Fourier transformation, and its inverse and Stehfest numerical inversion were employed to establish a point source and line source solutions. Furthermore, the validity of the proposed model was verified by comparing a field data from the Arum River Basin in Turkmenistan. Finally, the effects of major parameters on the production decline curves were analyzed by using the proposed model and it was found that they had influences at different stages of gas production history and the sensitivity intensity of each parameter was different. With its high efficiency and simplicity, this semianalytical model will serve as a useful tool to evaluate the well production behavior for the naturally fractured-vuggy carbonate gas reservoirs.

A new analytical model for flow in acidized fractured-vuggy porous media

Acidizing is one of the widely used technologies that makes the development of naturally fractured-vuggy reservoirs effective. During the process of acidizing, carbonate minerals are dissolved by hydrochloric acid, which can create high conductivity channels and wormholes to connect fractures and pores. In this work, a new analytical model, incorporating the heterogeneity of the pore networks into acidizing region, is proposed to study the flow characteristics in acidized fractured-vuggy reservoirs. The model is coupled by an acidized inner region and a conceptualized outer region of common triple medium. The porosity and permeability of inner region, which are rather heterogeneous and disordered when observed at different length scales, can be well addressed by fractal theory. The properties of the outer region can be described with three basic parameters: the matrix block size LM, the space interval of fracture LF and the radius of the vug Lv. Results show that the flow characteristic curves can be characterized by six flow stages (i.e. wellbore storage stage, radial flow stage in the interior region, fracture-vug inter-porosity flow stage, transition flow stage, fracture-matrix inter-porosity flow stage and external boundary response stage). It can be applied to estimate reservoir parameters for uncertainty reduction using inverse modeling.

A Seepage-Stress Coupling Model in Fractured Porous Media Based on XFEM

Large cracks are important seepage channels inside fractured-vuggy reservoirs. Therefore, in this thesis, the calculation method of fully coupled modeling of the fractured saturated porous medium based on the extended finite element method (XFEM) is established to study the expanding regularity of cracks in fractured-vuggy reservoirs. Fully coupled governing equations are developed for hydro-mechanical analysis of deforming porous medium with fractures based on the stress balance equation, the seepage continuity equation and the effective stress principle. The final nonlinear fully coupled equations reflect not only the coupling effect of the physical quantity within the porous medium but also the coupling between the medium and the fracture. During the spatial dispersion of coupled equations based on XFEM, two kinds of additional displacement functions are introduced in the displacement model of the fracture area to reflect the strong discontinuity of the fracture surface. The pore pressure enhancement function is also applied to represent the weak discontinuous features of the normal pore pressure. The validity and efficiency of this model and calculation are verified through three calculating examples. The following crack propagation laws are obtained: (1) The larger the water flow rate is, the longer the crack propagation length is, and the larger the propagation width is (2) The greater the crack angle and the crack length, the easier it is to expand the crack. Besides, compared with dip angle, the crack length has a more sensitive influence to the crack propagation. (3) When multiple cracks exist, the larger the fracture spacing is, the easier the crack will expand.

Experiments on nitrogen assisted gravity drainage in fractured-vuggy reservoirs

Abstract Visual models of fractured-vuggy reservoirs were designed and manufactured to conduct experiments of nitrogen assisted gravity drainage (NAGD). The impacts of flooding pattern, gas injection rate, well type, and displacement direction (vertical or horizontal) on development performances and remaining oil distribution were studied. The results show that during NAGD, the sweep scope is decided by the connections between producer and reservoir, and the local sweep efficiency is decided by fracture-vuggy configuration. The homogenous fractured reservoir has higher oil recovery, and the bigger the aperture of fracture is, the higher the recovery. The main regions of remaining oil due to poor connectivity and gas-oil gravity difference include blind fractures and vugs below the connected fractures, the bottom of vugs, and the narrow and low-angle fractures. The accumulation of remaining oil in the bottom of reservoir is easily formed and controlled by the connections between producers and reservoir. The higher the gas injection rate and the stronger the fracture heterogeneity, the earlier the gas channeling and the lower the oil recovery of the producer will be. Horizontal wells have the best development effect, so horizontal well can be applied in fractured-vuggy reservoirs without bottom water. Producers should be preferentially drilled at low structural position. Gas channeling firstly occurs in the producer at high structural position, and it should be shut in timely to improve the utilization of injected gas.

Analysis Method for Tracer Production Curve in Fractured-vuggy Reservoir

The fracture-vuggy carbonate reservoir in Tahe oilfield is unique and difficult to be analyzed by conventional tracer analysis technology. Based on the geological characteristics of fracture and vug in reservoirs, a theoretical model is established by applying fluent to simulate the flow characteristics of fluids in fracture-vug, so as to analyze the characteristics of tracers under different conditions: The output of tracer under the condition of single fracture shows kurtosis, while under the condition of single vug without kurtosis; The combination of many fractures can either enhance the peak height or show multiple peaks, while the combination of multiple vugs still shows no kurtosis but only extends the range; The output characteristics of series combination of fracture/vug were more influenced by themselves. The smaller the volume of vug is, the less the fractures were influenced, and thus the production becomes more and more similar to fractures; while the rest of them show vug characteristics. Through applying the tracer production curve analysis theory to the actual production data of the field tracer test, and establishing the fracture tunnel ratio and the dominant channel direction judgement method to characterize the reservoir flow path, we found that the ratio of the fracture and vug in the target area is 1:9; the direction of the dominant channel is determined by the size of the Interdistance intercepting area of the injection production well - the larger the section area is, the better the channel is. It indicates that the analytical method of tracer production curve in fracture-vug reservoir is applicable, and it can lay a foundation for improving the theory of tracer production characteristics analysis in fracture-vug reservoirs.

Lost circulation material for abnormally high temperature and pressure fractured-vuggy carbonate reservoirs in Tazhong block, Tarim Basin, NW China

Abstract To effectively solve the problem of lost circulation and well kick frequently occurring during the drilling of abnormally high temperature and pressure fractured-vuggy reservoirs in the Tazhong block, a rigid particle material, GZD, with high temperature tolerance, high rigidity (> 8 MPa) and low abrasiveness has been selected based on geological characteristics of the theft zones in the reservoirs. Through static pressure sealing experiments, its dosage when used alone and when used in combination with lignin fiber, elastic material SQD-98 and calcium carbonate were optimized, and the formula of a new type (SXM-I) of compound lost circulation material with high temperature tolerance and high strength was formed. Its performance was evaluated by compatibility test, static sealing experiment and sand bed plugging experiment. The test results show that it has good compatibility with drilling fluid used commonly and is able to plug fractures and vugs, the sealed fractures are able to withstand the static pressure of more than 9 MPa and the cumulative leakage is 13.4 mL. The mud filtrate invasion depth is only 2.5 cm in 30 min when the sand bed is made of particles with sizes between 10 mesh and 20 mesh. Overall, with good sealing property and high temperature and high pressure tolerance, the lost circulation material provides strong technical support for the safety drilling in the block.

Determination of Fracture Scale Limit in Numerical Simulations of Fractured Vuggy Reservoir

Determination of Fracture Scale Limit in Numerical Simulations of Fractured Vuggy Reservoir

Experimental study on waterflood development in large–scale karst structures

On the basis of the seismic data volume of a certain well group unit of the Tahe fracture–vuggy reservoirs, the similarity theory is adopted, and a numerical model with geometric, kinematic, dynamic, and fracture–cavity characteristic parameter similarities is designed. Moreover, a large–scale three–dimensional physical karst cave model is built with the carving technology. The model is used in the experimental study of two–phase flow of oil/water during waterflood development and in the analysis of relevant influencing factors. Results show that the displacement efficiency of low–high mode is obviously superior to the case of high–low mode. A high crude oil viscosity is found to equate to a low ultimate recovery. High–velocity water flooding improves ultimate recovery but results earlier water breakthrough. Well shutdown improves recovery factor and well shutdown during middle water cut stage achieves optimum effects. Bottom water energy benefits oil reservoir exploitation to some degree.

缝洞型油藏波动和流动耦合模型井底压力分析

Fractured vuggy reservoirs consist of matrix, fractures and vugs. Researchers usually treat the matrix-fracture-vug interaction as inter-porosity flow due to their complex pore system and flow mechanism.In fact, vugs play an important role in fractured vuggy reservoirs and can’t be simplified to one homogeneous medium. The idea that pressure spreading in vugs is in form of wave, which is similar to pressure decline, was proposed. Based on that, an analytical well test model for fractured vuggy reservoirs combined with seepage equations for outer formation was presented. Then the log-log type curves of the wellbore pressure and its derivative were obtained through the Laplace transform and numerical inversion. The results show that, the pressure curve forms were influenced by dimensionless parameters related to flow and wave propagation, also by dimensionless parameters related to the outer formation. Sensitivity analysis of these parameters were done. Lastly, a field example was demonstrated to validate the accuracy of the proposed model, which matched the real geological data well.

A heterogeneous model for simulating fluid flow in naturally fractured-vuggy carbonate reservoirs

In this paper, a heterogeneous model named matrix-fracture-vug (MFV), which considers the interactions of flow regimes in matrix, discrete natural fractures, large-scale fractures and vugs, is developed for simulating fluid flow in fractured-vuggy reservoirs. In the MFV model, coupling Darcy-Stokes equations are used to describe flow regime in matrix and vugs. Fluid flow follows the cubic law in each discrete natural fracture. An equivalent method is put forward in large-scale fractures. The flow mechanisms in fractured-vuggy reservoirs are analyzed, and the oil well productivity is demonstrated by investigating the influences of the matrix, fractures and vugs. The matrix permeability controls the production rate in the whole production process, and the conductivity of fracture network is another main influence factor. The cumulative production will increase due to the storage capacity of vugs in later period. This model can provide insights into the history matching and the production prediction for fractured-vuggy. [Received: August 4, 2016; Accepted: September 20, 2017]

Numerical simulation of fractured-vuggy porous media based on gamblets

Numerical simulation of a fractured-vuggy porous medium is a challenging problem. One reason is the coexistence of matrix, fractures and vugs on multiple scales that need to be coupled, and the other reason is that the high-resolution fractured-vuggy model may contain up to several millions of gridcells in applications, which brings severe computational challenges into the numerical methods. Therefore, the requirement for accurate and efficient technique is widely increasing. Fractured-vuggy porous medium is generally represented by triple-continuum model in which the matrix system, fracture system and vug system each are treated as a parallel continuous system. Although triple-continuum model is widely used because of its easy-implementation and high efficiency, it fails to capture the detailed flow patterns of reservoir with disconnected long fractures. Discrete fracture-vug network (DFVN) model can precisely model the fluid flow in fractures and vugs. However, the simulation of this model is deemed intractable even with the advent of supercomputers because of the large amount of calculation. In view of the fact that the multigrid method is now well known as one of the fastest method of solving elliptic problems, in this paper we introduce a nearly linear complexity multiresolution decomposition method for fluid flow in a fractured-vuggy reservoir. The detailed flow patterns are described by combing the advantages of continuum model and discrete model. That is, the homogenization theory is used to construct an equivalent permeability in each coarse grid block in which the vugs and small-scale fractures are represented by discrete fracture-vug network model. We decompose the solution space into several subspaces and then we compute the corresponding solutions of heterogeneous discrete fracture network model in each subspace. Gamblets are constructed and they are elementary solutions of hierarchical information games associated with the process of computing with partial information and limited resources. These gamblets have a natural Bayesian interpretation under the mixed strategy emerging from the game theoretic formulation. This method could realize its fast simulation by decomposing the solution space into a direct sum of linear subspaces that are orthogonal to each other. Finally, the pressure difference distribution of fractured-vuggy porous medium is obtained by combing the DFVN solutions of all subspaces. Numerical results are presented to demonstrate the accuracy and efficiency of the proposed multiresolution decomposition method. The results show that this method is a promising method of numerically simulating the fractured-vuggy porous medium.

Experimental study on different water flooding methods for fractured-vuggy reservoir based on 3D printing

To study the distribution of residual oil after water flooding and the effectiveness of improved water flooding, the 3D printer was used to print the 3D model to imitate the fractured-vuggy reservoir modles of Tahe Oilfield. The purpose of this paper is to quantitatively describe the distribution of the remaining oil, study the mechanism of different improved water flooding methods, and provide theoretical support and technical support for further study of oil recovery. The experimental results of the three-dimensional model show that the remaining oil in such fracture-cavity reservoirs mainly exists at the top of the structure and the corners of the cave model, as well as the injection wells and the raised centers of the model.The remaining oil at the injection well can be produced after the injection-production inversion. After gas injection, the oil at the top of the model and the raised centers can be produced, indicating that changing the water flooding method can increase the oil production of the well.

Effect of vug filling on oil-displacement efficiency in carbonate fractured-vuggy reservoir by natural bottom-water drive: A conceptual model experiment

Universal existence of the filled vug or cave in carbonate reservoir complicates the fluid flow as the coexistence of free flow and porous flow. The fills in vug increase the flow resistance and affect the water invasion for a natural bottom-water reservoir. This influence was explored through experimental tests on a conceptual discrete fractured-vuggy model. This model was abstracted from Tahe Oilfield in China, in the form of isolated vugs connected by fractures network. A new scaling method was proposed for this discrete media according to the similarity of the displacement front of water drive. It was found that vug filling displays a negative role during the natural bottom-water drive. This impact depends on the geological feature of fractured-vuggy reservoir. A stronger sensitivity occurred to the fracture well-developed stratum that the vug filling made the pre-existing or new preferential corridors more significant. As a result, the systems with multiple flow paths became more “heterogeneous”. In this condition, a lower water intensity and oil viscosity can ease these adverse effects.

Integrated geological-geophysical characterizations of deeply buried fractured-vuggy carbonate reservoirs in Ordovician strata, Tarim Basin

Abstract The Central Uplift region of the Tarim Basin contains a series of thick Ordovician carbonates that form fractured-vuggy hydrocarbon-rich reservoirs along an unconformity. These fractured-vuggy reservoirs are difficult to characterize using traditional well logging and seismic interpretation methods because these carbonates feature complex and heterogeneous lithologies and variations in scale, geometry and spatial distribution. In this paper, a series of deeply buried fractured-vuggy carbonate reservoirs are characterized on micro-, macro- and mega-scales. These characterizations are based on previous studies detailing the carbonate sedimentation, diagenesis, and tectonic processes in the study area and abundant geological and geophysical data, including core samples, thin sections, and well logging and seismic datasets, which have been obtained and upscaled using the “choosing-classification-evaluation- generalizing-upscaling” (CCEGU) method. In this area, different facies can be identified at different scales: (1) Detailed core and thin section observations performed based on the Dunham classification demonstrate that 7 lithofacies and 19 rock types are present at the micro-scale (0.01 m); (2) logging data and assorted reservoir characteristics indicate that these 19 different rock types consist of 9 different electro facies at the macro-scale (0.1 m–1 m), and the presence of electro facies in individual wells can be recognized; and (3) these 9 different electro facies comprise 6 distinct seismic facies at the mega-scale (10 m), and can be used to delineate the spatial outlines of potential reservoirs and evaluate their properties. The targets of particular interest in the study area are a series of partially dissolved dolomites located near large faults and a series of vuggy soluble limestones located near NE-trending strike-slip faults. This study combines geological and geophysical response data from fractured-vuggy carbonate reservoirs to achieve seamless connections among micro-scale, macro-scale and mega-scale observations to effectively characterize a series of strongly heterogeneous fractured-vuggy reservoirs. This method can also be applied to characterize other deeply buried paleokarst reservoirs in the Tarim Basin, as well as other strongly heterogeneous reservoirs.

New understandings of the seismic modes of high productivity wells in the Sinian Dengying Fm gas reservoirs in the Gaoshiti area, Sichuan Basin

The Sinian Dengying Fm in the Sichuan Basin is a set of fractured-vuggy reservoirs mainly dominated by sedimentation and karstification, and characterized by small fractures and vugs, complex stratigraphy and lithology, and strong vertical and lateral reservoir heterogeneity. As a result, the drilling rates are lower and single-well gas productivity varies greatly there. And the reservoir identification results based on seismic data in this area are ambiguous. In view of this, the Deng 4 Member in the Gaoshiti buried structure on the southern flank of the Leshan–Longnüsi palaeohigh was precisely divided in stratigraphy. Then, the types of reservoir assemblages were determined based on gas well logging and test data, and the seismic response characteristics of typical wells with different types of reservoir assemblages and the seismic modes of high productivity wells were studied by using high-resolution seismic data. It is indicated that the Deng 4 Member in this area is divided into three types of reservoir assemblages, which correspond to three seismic modes. Seismic mode I: It presents the seismic response characteristics of “wide wave troughs + double highlights” or “wide wave troughs + complex waves”, indicating that fractures and vugs are developed. It is the preferred seismic mode of high productivity wells in the stage of development. In this mode, the technology of highly deviated wells or horizontal wells can be implemented. Seismic mode II: It presents the seismic response characteristics of “wide wave troughs”, indicating that fractures and vugs are relatively developed. It is the mode of moderate productivity wells. In this mode, the technology of highly deviated wells can be implemented. Seismic mode III: It presents the seismic response characteristics of “wide wave troughs + highlights”, indicating that fractures and vugs are underdeveloped. It is mode of low productivity wells, in which the technology of horizontal wells can be implemented. The new understandings on seismic modes were applied on site. It is shown that the reservoir drilling rate is over 60% and the average gas production rate during the production test of 8 wells is up to 75.34 × 104 m3/d. It is concluded that the seismic mode of high productivity wells based on seismic facies, fracture and vug prediction and integrated target design provides a support for the well arrangement and drilling trajectory adjustment in natural gas exploration and development in the Gaoshiti area and good results have been achieved.

Research Progress of the Fractured-vuggy Reservoir Zones in Carbonate Reservoir

Research Progress of the Fractured-vuggy Reservoir Zones in Carbonate Reservoir

A Novel Semi-Analytical Model for Multi-branched Fractures in Naturally Fractured-Vuggy Reservoirs

In the past few decades, scholars have made great breakthroughs in the study of well test analysis of carbonate rock. The previous studies are based on horizontal wells, straight wells, fractured wells, and inclined wells. With the development of fracturing technology, acid fracturing technology is considered to be the most effective measure to develop carbonate reservoirs. As the carbonate rock is easily dissolved in carbonic acid, multi-branched fractures will be produced near a vertical well. This article presented a semi-analytical model for multi-branched fractures in naturally fractured-vuggy reservoirs for the first time, which laid a theoretical foundation for solving well test analysis for finite conductivity multi-branched fractures. The model can quantify the wellbore flow pressure and applied to obtain more parameters reflecting comprehensive flow characteristics through using history matching procedure. The results were compared with numerical simulation and the existing analytical solutions of a single fracture model. Then in this paper, flow characteristics are recognized and there are five flow regimes found in the type curves, e.g. bi-linear flow region, linear flow region, inter-porosity flow region between vugs and fractures, inter-porosity region between matrix and fractures, and radial flow region. Finally, the influence factors analysis shows fracture number will mainly affect flow behavior of bi-linear flow and linear flow. The angle analysis showed that as the fractures were closer, their interaction became stronger. The conductivity would seriously affect the flow behavior in the early time. Linear flow cannot be observed when the conductivity is less than 1 and bi-linear flow cannot be observed when the conductivity is more than 20. And the effect of fracture length on flow behavior occurs in the early time. Bi-linear flow and linear flow characteristics cannot be observed when the fracture length is reduced.

Application of water injection curves for the dynamic analysis of fractured-vuggy carbonate reservoirs

In Tahe Oilfield, Tarim Basin, NW China, the carbonate reservoir displays poor porosity and permeability in its matrix, while the fractured-vuggy system is distributed in a random, discreet and discontinuous way, which yields significant oil rates once the fractured-vuggy system is connected by production wells. The fractured-vuggy carbonate reservoirs are consisting of a group of large carves and inter-connected by high permeability fractures. Due to lack of powerful and suitable EOR (Improved Oil Recovery) strategies, depletion-drive recovery is commonly adopted for practical production at the beginning. After a period of production, the elastic energy of the cave formation weakens, causing insufficient liquid supply, the water injected for water flooding is used to improve the oil recovery of the fractured-vuggy carbonate reservoirs. However, the water injection curves of the carbonate reservoir are different from that of the sandstone reservoir as their fundamental flow mechanisms are different. For the fractured-vuggy reservoirs with high permeability, the water injection curves can be adopted to calculate the volumes in place of crude oil and formation water in a single or multi-caves. This paper presents a new model of injection curves. Using the proposed model with multi-round water injection curves, we can initially judge reservoir type and determine the volumes of cave, crude oil and water in the caves reasonably. Besides, as testified by the practical case, the parameters interpreted by the new models are proved to be reasonable and reliable. This new model is a useful tool to estimate the crude oil reserves, to demonstrate how much remaining oil in the caves, and to identify where the water-oil contact is.